The present invention relates to rotary fluid pressure devices, and more particularly, to such devices which are capable of two different ratios between the flow of pressurized fluid and the speed of rotation of the input-output shaft.
Although the present invention may be used with rotary fluid pressure devices having various types of displacement mechanisms, it is especially advantageous when used with a device including a gerotor gear set, and will be described in connection therewith.
It has long been an object of those skilled in the gerotor motor (or pump) art to provide a simple, but efficient, two-speed gerotor motor. As used herein, the term "two-speed" means that for any given rate of fluid flow into the motor, it is possible to select between two different motor output speeds, a high speed (accompanied by a relatively low torque), and the conventional low speed (accompanied by a relatively high torque).
U.S. Pat. No. 3,778,198 discloses the basic concept for achieving two-speed (or dual ratio) operation of a gerotor motor. The concept disclosed in the reference patent involves providing switchable valving, in addition to the normal rotary valving in the motor, such that one or more of the expanding volume chambers can be placed in fluid communication with the contracting volume chambers, rather than with the fluid inlet, thus effectively reducing the displacement of the gerotor gear set to increase the motor output speed for a given rate of fluid flow to the motor. This is referred to as the high speed, low torque mode. On the other hand, if all of the expanding volume chambers are placed in fluid communication with the fluid inlet, the motor operates in its normal low speed, high torque mode.
Although U.S. Pat. No. 3,778,198 successfully discloses and teaches the basic concept described above, the motor shown in the reference patent is of the spool valve type which, because of the fixed diametral clearance between the rotating spool valve and the adjacent cylindrical housing surface, have been limited to relatively lower pressures and torques. However, the market for a two-speed gerotor motor is primarily in connection with applications requiring relatively higher pressures and torques, and as of the filing of the present application, the motor shown in U.S. Pat. No. 3,778,198 has not been commercialized, nor is any other two-speed gerotor motor commercially available.